Manual and automated supercritical fluid extraction and chromatography systems require orifice restrictors for regulating the flow of fluid during extraction and chromatography operations. It is conventional in such apparatus to employ fixed orifice restrictors. These restrictors which are typically made of stainless steel, fused silica or ceramic are unsuitable since they tend to clog, break and/or require frequent maintenance. Furthermore, they are limited to a fixed flow rate at a given pressure/density setting thus significantly reducing the versatility of the extractor and chromatography systems.